1. Technical Field
This invention relates to the routing of telecommunications signals. More particularly it relates to a method of routing such signals to both fixed and mobile telecommunications mediums, such that similar services can be used in the same way by users on either medium, and to allow system operators to reduce costs by greater commonality of switching and other network-based facilities. The present invention is concerned with the routing of packet-based communications such as those used in the “Internet” using the so-called “Internet Protocol” (IP).
2. Description of Related Art
Present mobile medium systems are arranged such that a mobile user and associated systems collaborate at the interface with the network (typically the radio base station) to enable a mobile node to change from communicating with one base station to communicating with another, and to enable the network to update intelligence points of the new location. In cellular networks, these intelligence points are the Home and Visitor Location Registers (HLR and VLR), whilst in “Mobile IP” these locations are known as the Home and Foreign Agent. In both cases the “Visitor” Location Register or “Foreign” Agent maintains a record only of those users currently co-operating with base stations under their supervision, whilst their “Home” counterparts maintain a permanent record of their associated users, including a record of which VLR or Foreign Agent each one is currently working with. The address on an incoming message identifies the relevant HLR/Home Agent, to which reference is made to identify the appropriate VLR/Foreign Agent for more specific routing details. This allows minor changes in location to be effected within the VLR/Foreign Agent, locally to the user's current location without informing the HLR/Home Agent, which could be some distance away, thereby greatly reducing the signalling overhead.
The additional cost of mobility is the provision of this Home Agent/Foreign Agent interface, and especially with packet systems, the cost of tunnelling (forwarding messages from one address to another), address exhaustion (the inability to re-use an address from which forwarding is taking place) and triangular routing.
In a fixed medium system, IP routing is based on the distribution of IP address blocks or prefixes, with an associated metric or route cost, from potential destinations to potential senders so that they and intermediate routers can determine the best next hop (neighbour router) towards that destination. These routes are pre-computed for all destinations in the network so that senders can immediately send information when generated. Pre-computation of routes, and deployed routing exchange technology, is possible when the sources and destinations have a fixed location, and communication bandwidth is rich enough for exhaustive exchange of routes. As the proportion of roaming increases however, such models break down and a more dynamic routing approach is required.
A proposal referred to as “HAWAII” was published 19 Feb., 1999 as an Internet-draft entitled “IP Micro-Mobility Support Using HAWAII”, R. Ramjee, T. La Por, S. Thuel, K. Varadh, posted on the Internet Engineering Taskforce Internet (HTTP:) site at www.ietf.org/internet-drafts/draft-rimjee-micro-mobility-hawaii-00.txt. HAWAII uses specialised path set up schemes which install host-based forwarding entries in specific routers when in a routing domain to support inter-domain to support intra-domain micro-mobility, and defaults to using “Mobile-IP” for inter-domain micro-mobility. In HAWAII, mobile hosts retain their network address while moving within the domain. The HAWAII architecture relies on a gateway router into a domain, referred to as the domain root router, to which default routes within the domain are directed. Each mobile host is assigned a home domain based on its permanent IP address. The path set up scheme updates a single routing path in a domain so that connectivity to the mobile host is possible both before and after handoff at the wireless link layer. Only routers located along a single routing path between the domain root router and the base station currently serving the mobile host have routing table entries for the mobile host's IP address. The remainder of the routers in the domain route any packets addressed to the mobile host upwards along default routes which rely on the tree-like nature of the routing domain, rooted at the domain root router, to provide an intersection with the downrouting towards the mobile host along the single routing path for which the routers have individual host entries for the mobile host's IP address.
In HAWAII, mobility between domains is supported by “Mobile IP” mechanisms. The home domain root router is designated as the Home Agent, and encapsulated IP packets are forwarded via the Foreign domain root router.
Drawbacks with the HAWAII proposals include the concentration of Mobile IP tunnels in few nodes in the core of the network, the domain root routers, such that failure of any of these nodes may result in large-scale failure of all Mobile IP state and associated sessions handled by the failing node. Furthermore, since all routing from outside the home domain into the home domain, and in the reverse direction, must occur via the home domain root router, failure of the home domain root router may also result in large-scale failure.